The cabin of a modern passenger aircraft is air-conditioned usually both when the aircraft is flying and is on the ground by means of the aircraft's air conditioning system. The aircraft air conditioning system is supplied with pressurized bleed air taken from a compressor of an aircraft engine, such as a turbofan, or the compressor in the aircraft's Auxiliary Power Unit.
Typically, the bleed air passes through an “air cycle machine” that comprises, among others, one or several heat exchangers, a compressor and one or several turbines that cool the bleed air to a desired lower temperature and pressure. In this type of air conditioning system, the air exiting the air cycle machine follows an inverse Brayton cycle and is usually mixed in a mixing chamber with air recirculated from the cabin. Pipes exiting the mixing chamber may receive air at different temperatures from a manifold, in such a way that they blow air into the cabin at different temperatures.